This invention relates to polymeric materials for solar cell applications and methods for improving properties of low cost polymeric materials for solar cell applications. More particularly, this invention relates to low cost polymeric materials with improved properties for solar cell applications and methods for improving the properties of such polymeric materials for solar cell applications while maintaining their low cost.
Polymeric materials are commonly used in the manufacture of solar cell modules. Specifically, polymeric materials are principally used as encapsulants for solar cells and as backskin materials in solar cell modules. For crystalline silicon solar cells, the cells can be encapsulated such that a transparent encapsulant is used between a transparent superstrate (usually glass) and the solar cell. In this case, a second layer of encapsulant, which may be pigmented, can be used between the solar cells and the backskin material. For thin film solar cell modules (e.g., amorphous silicon, cadmium telluride, or copper indium diselenide) a single layer of encapsulant is employed.
With the heightened interest in solar cell modules in architectural applications, there has developed an even greater need than ever before for materials which are used in solar cell modules that can show high resistance to thermal creep at temperatures as high as 90xc2x0 C. Such temperatures have been reached in some architectural applications. temperatures have been reached in some architectural applications.
As used herein, the term xe2x80x9cthermal creepxe2x80x9d refers to permanent deformation of a polymer effected over a period of time as a result of temperature. Thermal creep resistance, generally, is directly proportional to the melting temperature of a polymer. For materials with low melting temperatures, it is necessary to cross-link the materials to given them higher thermal creep resistance.
Polymeric materials commonly used as transparent encapsulants include thermoplastics such as ethylene vinyl acetate (EVA) and ionomers. EVA, the most commonly used material, is a co-polymer of vinyl acetate and ethylene. Ionomers are copolymers of ethylene and methacrylic acid with a salt added to neutralize them. Ionomers can be used alone or with metallocene polyethylene. Metallocene polyethylene can be a copolymer (or comonomer) of ethylene and hexene, butene, or octene. Encapsulant materials comprising both ionomers and metallocene polyethylene are described in a commonly owned U.S. patent application, entitled xe2x80x9cEncapsulant Material for Solar Cell Module and Laminated Glass Applications, Ser. No. 08/899,512, filed Jul. 24, 1997 now U.S. Pat. No. 6,140,046.
These polymeric materials each address the thermal creep resistance problem in a different manner. For EVA, which has a rather low melting point, chemical cross-linking is used to provide thermal creep resistance. An organic peroxide is added to the EVA and cross-links it using the heat of a lamination process. A problem with this chemical cross-linking procedure is connected with the fact that total cross-linking is not fully achieved. Therefore, the peroxide used as the cross-linking agent is not completely used up during the process and excess peroxide remains in the laminated EVA. The remaining peroxide can promote oxidation and degradation of the EVA encapsulant. Also, the addition of some organic peroxides to the EVA sheet extrusion process require stringent temperature control to avoid premature cross-linking in the extruder chamber. This makes EVA with this peroxide addition difficult to manufacture into sheet.
Ionomers do not require chemical cross-linking agents. Instead, thermal creep resistance is provided by the built-in cross linking which the ionically bonded regions in the ionomers provide. Metallocene polyethylene can have melting temperatures of about 100xc2x0 C., anywhere from 5 to 15xc2x0 C. higher than ionomers and about 40xc2x0 C. higher than EVA which has not been cross-linked. Thus, they can exhibit better thermal creep resistance simply by virtue of their higher melting temperature. However, even higher creep resistance may be called for than presently exists for any of these encapsulants.
Polymeric materials commonly used as backskin layers include a Tedlar (i.e., a DuPont trade name for a type of polyvinyl difluoride) laminate, polyolefins and polyolefin mixtures. Backskin layers of solar cell modules require thermal creep resistance at temperatures considerably greater than 90xc2x0 C. to satisfy a certification test known as the Relative Thermal Index (RTI). Thermal creep resistance at temperatures at or above about 150xc2x0 C. are called for to satisfy the RTI test. The Tedlar laminate now widely used does satisfy the RTI requirement but it has other limitations. It is expensive, requires an additional edge seal and is thin, the entire laminate being about 0.010 inches thick and the Tedlar only about 0.002 inches thick.
Polymeric materials that can be used as backskin layers to form frameless modules should also exhibit thermoplastic properties during lamination, which is typically performed at temperatures on the order of 140xc2x0 C.-175xc2x0 C. One type of frameless solar cell module is described in commonly owned U.S. patent application, entitled xe2x80x9cSolar Cell Modules with Improved Backskin and Methods for Forming Same, Ser. No. 08/671,415, filed on Jun. 27, 1996 now U.S. Pat. No. 5,741,370. For such modules, the backskin material must be capable of being softened, molded and formed during lamination (i.e., it must exhibit enough thermoplastic behavior to allow for this, while still exhibiting enough thermal creep resistance to satisfy RTI requirements).
The present invention features methods of manufacturing solar cell modules with improved thermal creep resistance. In one aspect, polymeric materials used in the manufacture of solar cell modules are subjected to electron beam radiation, which cross-links the polymeric materials without entirely eliminating their thermoplastic properties. In one embodiment, a backskin layer is irradiated by a high energy electron beam before being used to form the solar cell module, resulting in the backskin material having vastly improved thermal creep resistance and the capability to pass an RTI test. In another embodiment, one or more transparent encapsulant layers are irradiated by a high energy electron beam to increase their thermal creep resistance and then used to form a solar cell module.
In another aspect, the invention features methods of interconnecting solar cells which have wraparound contacts. A metal interconnection pattern is placed on a backskin layer that is predisposed to electron beam radiation. The backskin layer with the metal interconnection pattern is placed adjacent rear surfaces of the solar cells. The cells can then be interconnected by merely being placed down on this backskin material and bonded using either a conductive epoxy or by being soldered. In this way, a monolithic module can be formed. Thermal creep resistance at T≅ is called for here and can be obtained using the results of this invention.
In yet another aspect, the invention features a solar cell module with improved thermal creep resistance. The solar cell module includes a transparent superstrate, at least one transparent encapsulant layer, interconnected solar cells and a backskin layer. Either or both of the transparent encapsulant layer and the backskin layer are predisposed to an electron beam radiation to form highly cross-linked polymers, while still maintaining some thermoplastic qualities.
In still another aspect, one or more polymeric bonding layers that are predisposed to an electron beam radiation seal components to a solar cell module. By irradiating the polymeric bonding layers, the polymeric material is cross-linked while still maintaining the surface bonding properties of the layers. In one embodiment, a backskin layer of a solar cell module wraps around edges of the solar cell module to form an edge seal. A bonding layer that is predisposed to electron beam radiation is placed in between the backskin layer and a front surface of a transparent superstrate to bond the wrapped portion of the backskin to the front support layer.
In another embodiment, a bonding layer that is predisposed to an electron beam radiation seals electrical leads to a rear surface of the backskin. In yet another embodiment, a bonding layer that is predisposed to an electron beam radiation bonds mounting elements to a backskin of a solar cell module. In yet another embodiment, polymeric layers which are used to electrically isolate current carrying leads vertically disposed relative to each other but separated by these polymeric layers are electron beam irradiated to further enhance their electrical insulation properties.